Controllers for touchscreen devices send the position-dependent signals generated by the touchscreens to analog to digital converters (ADCs) to be digitized before further processing, including digital filtering, is carried out, to extract the positional information of interest. Touchscreen devices are often operated in environments where large amplitude pulses or bursts of RF noise may be encountered. Such impulsive RF noise events may be periodic or aperiodic, with switching power supplies or fluorescent lighting systems being examples of possible sources of the periodic type, and cell phone transmission events being possible sources of the aperiodic type. In some cases, the noise may be to some extent predictable.
When the linear digital filters normally present in the circuitry that processes the digitized signals encounter impulsive noise, the filters typically respond by “ringing”. This response may disrupt normal operation for the duration of the ringing, by masking underlying touch signals, and/or by providing signals indicating non-existent i.e. false touches. Either outcome is clearly undesirable for the user.
What is needed is a touchscreen system, and method of operating such a system, that can detect or accurately predict the occurrence of impulsive noise bursts, prevent the filter ringing that would occur if such noise were processed by the digital filters of that system, and return the system to normal operation in as short a time as possible, so that accurate touch tracking is resumed.